This invention relates to coatings for metallic substrates which are based on epoxy resins as well as to methods of preparing dispersions suitable for forming such coatings through autodeposition.
Over the last few decades, various water-based coatings for metallic surfaces have been developed which are commonly referred to in the field as autodeposition coatings. Such coatings utilize dispersions of resins capable of forming a protective coating when cured. The coating typically is applied by immersing the metallic surface in a bath containing the resin dispersion, acid, and an oxidizing agent to form an adherent coating that is initially wet. The thickness of the coating can be affected, for example, by such factors as total solids, pH and oxidant or accelerator concentration. Further, the coating thickness is a function of the immersion time. The initial wet coating is sufficiently adherent to remain attached to the surface on which it is formed against the influence of normal gravity and, if desired, can be rinsed before being cured (i.e., converted to a dry, solid and even more adherent coating) by heating.
A number of different resins have been proposed for use in autodeposition coatings, as described, for example, in U.S. Pat. Nos. 3,063,877; 3,585,084; 3,592,699; 3,647,567; 3,791,431; 4,030,945; 4,186,226; 3,795,546; 4,636,265; 4,636,264; and 4,800,106, each of which is incorporated herein by reference in its entirety.
Epoxy resin-based autodeposition coating systems are described in U.S. Pat. Nos. 4,233,197; 4,180,603; 4,289,826; and 5,500,460 and in International Publication Number WO 97/07163 (corresponding to U.S. Ser. No. 60/002,782, filed Aug. 16, 1995 and U.S. Ser. No. 09/011,869, filed Aug. 7, 1996), the teachings of each of which are incorporated herein by reference in their entirety. However, the epoxy resin-based autodeposition coatings developed to date generally have one or more shortcomings which have limited their usefulness, such as, for example, inconsistent coating appearance and quality.
This invention provides a metallic substrate having a coating adhered thereto, said coating being obtained by autodeposition and comprising an adduct of at least one epoxy resin which is a glycidyl ether of a first polyhydric phenol and at least one reactant containing at least one flexibilizing segment.
This invention also provides a composite comprised of the aforementioned coated metallic substrate, a rubber substrate, and an adhesive bonding said metallic substrate to said rubber substrate.
Additionally, this invention provides a method of making an aqueous dispersion suitable for use in forming an autodeposited surface coating, said method comprising
a) contacting at least one epoxy resin (wherein preferably at least one epoxy resin is a flexibilizing epoxy resin) and at least one chain extender containing at least two epoxy-reactive groups such as hydroxyl, carboxylic acid, carboxylic acid anhydride, and combinations thereof for a time and at a temperature effective to accomplish reaction of said epoxy resin and chain extender to form an adduct having an increased molecular weight as compared to the epoxy resin;
b) emulsifying a solution of the adduct (and, preferably, at least one cross-linker capable of reacting with functional groups present in the adduct) in an organic solvent with water in the presence of an emulsifier to form an organic solvent-containing intermediate emulsion; and
c) removing the organic solvent from the organic solvent-containing intermediate emulsion to form the aqueous dispersion.
This invention also provides an autodeposition bath composition comprising (a) at least one of the aforedescribed adducts, (b) at least one emulsifier, (c) at least one cross-linker, (d) at least one accelerator component such as acid, oxidizing agent and/or complexing agents, (e) optionally, at least one colorant, (f) optionally, at least one filler, (g) optionally, at least one coalescing agent, and (h) water.
Also furnished by this invention is a method of coating a metallic substrate comprising the steps of contacting said metallic substrate with the aforedescribed autodeposition bath composition for a sufficient time to cause the formation of a film of the dispersed adduct particles on a surface of the metallic substrate, separating the metallic substrate from contact with the autodeposition bath composition, rinsing the metallic substrate, and heating the metallic substrate to coalesce and cure the film of the dispersed adduct particles adhered to said surface.